FIG. 1 of the accompanying drawings illustrates the general design of a linear reflector 12 based on a linear light source 11, which is widely used in general lighting, automotive headlamps, etc. The linear reflector 12 is parabolic in its cross section. The linear light source 11 is located at the focal point of the parabolic shaped linear reflector 12. The light source 11 emits light in all directions. When the light is reflected by the parabolic reflector 12 it is collimated in the z-y plane. The light, however, is still uncontrolled regarding beam shaping in the z-x plane. The angular distribution of the output light measured by a detector plane 13 is shown in FIG. 1(b).
On the other hand, in order to increase the light output, the reflectance of the reflector 12 needs to be as high as possible, or the light loss on the reflector 12 needs to be minimized. In order to do so, higher cost is normally associated which becomes an issue and a hundred percent reflectance is still impossible.
U.S. Pat. No. 6,760,157 B1 (R. C. Allen) describes a design for a brightness enhancement film (BEF) available from 3M. Using the microstructure suggested in this patent as represented in FIG. 2, light 22 with a wide emitting angle can be compressed close to a normal angle. At the same time, light 25 emitting with a normal angle will be reflected by total internal reflection (TIR) within the film 21 and then recycled 24 inside the system by diffuser sheet or reflector 23 below it. As a result, with one BEF the angular distribution of the output light is improved which means the peak brightness is increased in one direction (for example, left and right). With another BEF that has its elongate structure perpendicular to the previous BEF the light output angular performance can be further improved in the other direction (for example, back to front). The BEFs are used widely in backlighting systems especially with liquid crystal display (LCD) systems because LCD panels perform better when the light passes therethrough close to the normal incident angle.
U.S. Pat. No. 6,161,946 (C. B. Bishop) describes a reflector design for a lamp as represented in FIG. 3. The reflector consists of an elliptical reflector 35 and a spherical reflector 32 to get the best focused beam. In this design, the light source 31 is located on one of the focal points of the elliptical reflector 35; therefore the light coming from the source and being reflected by the elliptical reflector 35 will be focused to the conjugate focal point. Light traveling directly away from the elliptical reflector 35 will be reflected by the spherical reflector 32. Because the location of the light source 31 is also the central point of the spherical reflector 32, the light hitting the spherical reflector 32 will be reflected back to exactly the same path then get reflected again by the elliptical reflector 35 and focused on the same conjugate focal point. Therefore, the best focused beam is obtained. There are three main limits in the design. The first two relate to the reflector coating process and performance. Higher reflectance normally means higher cost with respect to the coated material and process. which This results in a cost issue. Also, light loss which takes place on the reflector is generally around 5-10% and will cause thermal issues as well. The third limit is that this circular type reflector design does not work well with the linear type light source.